api-ref: avoid mushy wording around server.image description
api-ref: avoid mushy wording around server.image description

This disambiguates the description on the server.image parameter.
If the server is booted from volume (where boot from volume means
there is a bdm with boot_index=0 and destination_type=volume),
then the server.image will be an empty string [1] otherwise it
will be a dict.

[1] https://github.com/openstack/nova/blob/2f0f8b8cf13b7441ce6a7a524b33d30d40f1d468/nova/api/openstack/compute/views/servers.py#L540

Change-Id: Ia1ecd39a5636fa5a35cdbcafc03828b2925a9033

openstack/nova,openstack/nova,klmitch/nova,rahulunair/nova,mahak/nova,rahulunair/nova,mahak/nova,rahulunair/nova,klmitch/nova,klmitch/nova,klmitch/nova,mahak/nova,openstack/nova